U.S. President Barack Obama delivers remarks at the Edmund Pettus Bridge in Selma, Alabama, March 7, 2015. REUTERS/Jonathan Ernst When it comes to President Barack Obama exerting presidential powers, Republicans have made it clear they want to keep him in check.

Except when they don't.

Democrats, for their part, want the president to be aggressive in his use of executive authority.

But not at all times.

Obama's go-it-alone strategy on immigration, the environment and other policies has been a recurring target of the GOP, which risked an agency shutdown in an effort to restrain the president.

But on issues such as trade and fighting Islamic State militants, Republicans want Obama to have more authority, not less.

Democrats are their foil, cheering Obama's actions on immigration and the environment, but eager to rein in the president on matters of trade and war.

Obama is not the first president to want to flex his muscle and then find congressional pushback. But the lack of consistency between the parties was apparent as Democratic lawmakers resisted Republican efforts to curtail Obama's actions on immigration, but flipped sides as the Senate prepared for a hearing on Obama's proposal to confront IS.

Republicans also are working to build support for giving Obama special authority to negotiate international trade deals that Congress can only approve or reject, not amend. Most Democrats are fighting back.

Obama with Rhodes, U.S. Ambassador to the U.N. Samantha Power and Secretary of State John Kerry in New York on September 25, 2014. REUTERS/Kevin Lamarque "In the main, what you see is Democratic legislators who will call for and embrace expansive presidential powers as long as a Democrat occupies the White House. And likewise with Republicans," said University of Chicago political scientist William Howell, co-author of the book, "Thinking about the Presidency: The Primacy of Power."

"What's interesting that's going on now is that you have some policy domains where Republicans are actually arguing on behalf of stronger presidential power while a Democrat is in office," he said.

Obama's request for authority to use military force against IS a case study in the president trying to thread the partisan needle in Congress. He is asking to be barred from making a sustained commitment of U.S. ground forces and that the use of force authority would expire after three years.

Many Republicans want broader authorization, with no limits on troops. Democrats want a narrower authorization than what Obama proposed, limited to training and equipping local forces and conducting airstrikes.

In an interview last week, Senate Minority Leader Harry Reid, D-Nev., voiced doubts that Obama's split-the-difference approach would yield new military authority for the president because "the sweet spot has not been reached on that legislation."

AP Even without new authority, Obama could rely on the authorization obtained by President George W. Bush in 2001 that granted the president powers to pursue terrorists.

Reid, for one, has a more expansive view of the president's military powers than many of his Democratic colleagues.

"I think the president has the power to do basically whatever he wants," Reid said. "I'm in the minority, but that's how I feel. And not only this president, any president."

On trade, presidents traditionally have obtained the "fast-track" authority Obama is seeking, known as Trade Promotion Authority, but have faced opposition from a majority of Democrats. These Democrats maintain that unencumbered commerce can cost the U.S. jobs and that a quicker voting process restricts Congress' ability to influence an important part of American economic policy.

Republicans and the other wing of Democrats on trade say the conditions Congress would place on the president do ensure a significant voice for lawmakers.

"I am perplexed by arguments some make that TPA (Trade Promotion Authority) gives away Congress' power," GOP Sen. Orrin Hatch of Utah, chairman of the Senate Finance Committee, recently told the conservative American Enterprise Institute. "The reality is quite the opposite — TPA empowers Congress, expanding and enhancing its role in ongoing international trade negotiations."

There is a difference, for sure, between Obama requesting expanded authority from Congress and simply using his powers unilaterally as he did on immigration, the environment and on putting his health care law in place.

President Barack Obama announces his immigration plans from the White House. YouTube

It's not as if Obama's powers have not been tested. The Supreme Court last year declared that three Obama appointments to the National Labor Relations Board were unconstitutional because Obama sidestepped the Senate while it was officially in session.

House Republicans have sued the Obama administration, accusing it of exceeding its constitutional powers in carrying out Obama's health care law. Last month, a federal district court judge issued an injunction stopping the administration from implementing actions that would give work permits to certain immigrants who are in the country illegally.

Presidential efforts to test the limits of power are not new. And once exercised, barring judicial intervention they are hard to restrict.

"The president doesn't give back that which was given to him before," Howell said. "What you see over the long arc of history is, if not a steady, a dramatic expansion of presidential power and authority."

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